It is known that rods and tubes composed of metal can be produced by a continuous metal casting method as disclosed in U.S. Pat. No. 4,414,285. In this method, molten metal is upwardly supplied to a casting vessel (mold) where the molten metal is exposed to an alternating electromagnetic levitation and containment field which forms it into a column shape while being moved upwardly in the casting vessel. Simultaneously, the molten metal column is sequentially cooled and solidified, and the solidified metal product thereafter is removed from the top of the casting vessel by withdrawal rolls. This electromagnetic levitation type continuous metal casting method has been practically used as an industrially effective means of production. According to the aforementioned electromagnetic levitation type continuous metal casting method, a molten metal column to be cast or formed can be readily removed free from frictional forces and bonding forces against the sides of a casting vessel because the aforementioned alternating electromagnetic levitation and containment field produces a gravity-free state referred to as "pressureless contact." In addition, in such a method, while the molten metal column passes through the alternating electromagnetic field, the inside of the molten metal column is stirred and thereby high homogeneity can be accomplished.
A known prior art apparatus using the aforementioned continuous metal casting method is shown in FIG. 1. This apparatus comprises a molten metal storing furnace 2 for storing and holding a molten metal 1 and a tube-shaped casting vessel 3 vertically disposed for receiving the molten metal in the form of a column. So as to solidify the molten metal 1, a heat exchanger means 4 is unified with the casting vessel 3 for cooling and solidifying the molten metal column received into the casting vessel 3. An alternating electromagnetic field generation means 5 composed of a plurality of coils is disposed around the periphery of the casting vessel 3 and heat exchanger 4 for generating an alternating electromagnetic levitation and containment field that acts on the upwardly moving molten metal column. A means 6 such as withdrawal rolls is provided for removing the solidified metal product which has been cooled and solidified from the top of the casting vessel 3. A molten metal supply path 7 is provided for upwardly supplying the molten metal to be cast from the molten metal storing furnace 2 into the casting vessel 3. The molten metal supply path 7 is a tube formed of graphite or some similar refractory material with a high frequency heating means 8 disposed on the periphery thereof. Finally, a liquid level adjusting unit 9 is provided for adjusting the liquid level of the molten metal 1.
After extensive experience in operating a prior art production system using the aforementioned electromagnetic levitation type continuous metal casting method, several problems were encountered.
As one of the problems, the molten metal supply path 7 for upwardly supplying the molten metal 1 to be cast from the molten metal storing furnace 2 into the casting vessel 3 should continuously supply the molten metal 1 while keeping it in a molten state. For this purpose, a supply pipe with high conductivity material (such as graphite or other refractory material) is used and the high frequency heating means 8 is disposed around the periphery thereof. However, the molten metal supply path 7 extends horizontally to the casting vessel 3, which is vertically disposed, and requires an elbow section 7b. It is difficult to structure the high frequency heating means 8 around the elbow 7b and thus, the molten metal 1 cannot always be kept in the molten state. As a result, when the molten metal is supplied at the relatively low speed required when performing a low speed casting operation, the molten metal being supplied becomes partially cooled and tends to solidify at the bend section 7b, and the required amount of the molten metal 1 cannot be continuously supplied. Thus, in the molten metal supply path 7, an improvement of the apparatus for continuously supplying molten metal 1 is needed.
In the known electromagnetic levitation type continuous metal casting apparatus as shown in FIG. 2, which is an enlarged sectional view of the principal portion of the casting vessel of FIG. 1, the casting vessel 3, the heat exchange means 4, and the alternating electromagnetic field generation means 5 are formed in that order. The tube-shaped casting vessel 3 has a refractory-type heat conducting layer 3a, such as a graphite liner or the like, disposed on the inner wall thereof, and a flow path 4a of a cooling means (heat exchange means) surrounds layer 3a. In addition, around the full length of the outer periphery of the flow path of the cooling (heat exchange) means 4, a plurality of electromagnetic levitation coils (alternating electromagnetic field generation means) 5 are disposed. In such a structure, if the alternating electromagnetic field generation means 5 is composed of six sections of coils 5a, required full strength of the levitation electromagnetic field is obtained in the area adjoining the second section from both the ends thereof.
However, in the aforementioned electromagnetic levitation type continuous metal casting apparatus, there is the following problem. With reference to FIG. 1, the problem occurs as the molten metal column is supplied upwardly from the molten metal storing furnace 2 through the molten metal supply path 7 into the bottom of the casting vessel 3 where it is cooled and solidified by the heat exchange means 4. At that time, the molten metal column is electromagnetically and upwardly levitated by the alternating electromagnetic field generation means 5 and desired cast products, such as rods, are continuously produced. During this process, the rod often breaks. This is due to the fact that part of the molten metal column supplied upwardly to the casting vessel 3 is partially solidified in the area adjoining the lower coil 5a1, which is the first coil section from the bottom of the alternating electromagnetic field generation means 5, namely the area where levitating force and inwardly directed containment force cannot be satisfactorily obtained. Thus, the molten metal column is in contact with a cooled portion of the wall of the casting vessel 3, thereby disturbing smooth upward movement of the molten metal column. In an effort to solve such a problem in the wall area of the casting vessel 3 adjacent to coils 5a1 and coil 5a2, which are respectively the first and second coils from the bottom, a ceramic tube 3b is disposed and an air gap is disposed in the wall of the casting vessel 3 so as to decrease the thermal conductivity. However, in the aforementioned structure, the addition of these elements has not fully solved the problem.
Another problem is with respect to the molten metal supply path. In the prior art as shown in FIG. 3, an apparatus with a displacer 9 has been used. The displacer 9, upon being immersed in the molten metal 1 in the molten metal storing furnace 2, functions to supply the molten metal in the molten metal storing furnace to the casting vessel 3 through the molten metal supply path 7. In this prior art structure, the molten metal supply path 7 is connected to a side wall in the vicinity of the bottom of the molten metal storing furnace 2. The molten metal supply path 7 is composed of a horizontal section 7a, a vertical section 7c, and the bend section 7b for connecting them. In this case, the molten metal supply path 7 for upwardly supplying the molten metal 1 to be cast from the molten metal storing furnace 2 into the casting vessel 3 is generally composed of a graphite tube with high thermal conductivity and a heating means, using high frequency heating coils or the like, disposed around the outer periphery of the graphite tube. The graphite tube is readily oxidized and eroded by oxygen in the air or the molten metal 1. In other words, the durability of the graphite tube is low. Additionally, there are many joints between the horizontal section 7a and the molten metal storing furnace 2, between the horizontal section 7a and the vertical section 7c and between the vertical section 7c and the connecting bend section 7b. Repair and replacement of the joints are complicated, time consuming, and costly.
The possibility of the leakage of the molten metal 1 at such joints is increased further by the hydrostatic pressure produced by the molten metal 1 during the casting operation. In addition, in this prior art apparatus, repairing and replacing the-casting vessel or the molten metal transfer tube sections requires that the molten metal 1 in the molten metal storing furnace 2 be removed along with that in the casting vessel 3 and supply path 7. Such removal wastes time, some raw materials, and increases the cost of producing products. Therefore, an object of the present invention is to provide an electromagnetic levitation type continuous metal casting apparatus that facilitates decreasing or preventing the leakage of the molten metal 1 through the molten metal supply path 7. This results in an electromagnetic levitation type continuous metal casting apparatus free of both the need for complicated repair and too much replacement of joints in the molten metal supply path 7 together with the attendant loss of molten metal 1 flowing in the supply path 7 from the molten metal storing furnace 2. Additionally, it makes possible the repair and replacement of the casting vessel and molten metal supply path parts without requiring that the molten metal storing furnace be drained.